Caving to public pressure, Beijing environmental authorities started releasing more detailed air quality data Saturday that may better reflect how bad the Chinese capital's air pollution is.

The initial measurements were low on a day where you could see blue sky. After a week of smothering smog, the skies over the city were being cleared by a north wind.

The readings of PM2.5 - particulate matter less than 2.5 micrometers in size or about 1/30th the average width of a human hair - were being posted on Beijing's environmental monitoring center's website. Such small particulates can penetrate deep into the lungs, so measuring them is considered a more accurate reflection of air quality than other methods.

It is the first time Beijing has publicly revealed PM2.5 data and follows a clamor of calls by citizens on social networking sites tired of breathing in gray and yellow air. The U.S. Embassy measures PM2.5 from a device on its rooftop and releases the results, and some residents have even tested the air around their neighborhoods and posted the results online.

Beijing is releasing hourly readings of PM2.5 that are taken from one monitoring site about 4 miles (7 kilometers) west of Tiananmen Square, the monitoring center's website said Saturday. It said the data was for research purposes and the public should only use it as a reference.

The reading at noon Saturday was 0.015 mg/m3, which would be classed as "good" for a 24-hour exposure at that level, according to U.S. Environmental Protection Agency standards. The U.S. Embassy reading taken from its site on the eastern edge of downtown Beijing said its noon reading was "moderate." Its readings are posted on Twitter.

Steven Andrews, an environmental consultant who has studied Beijing's pollution data since 2006, said he was "already a bit suspicious" of Beijing's PM2.5 data. Within the 24-hour period to noon Saturday, Beijing reported seven hourly figures "at the very low level" of 0.003 milligrams per cubic meter.

"In all of 2010 and 2011, the U.S. Embassy reported values at or below that level only 18 times out of over 15,000 hourly values or about 0.1 percent of the time," said Andrews. "PM2.5 concentrations vary by area so a direct comparison between sites isn't possible, but the numbers being reported during some hours seem surpisingly low."

The Beijing center had promised to release PM2.5 data by the start of the Chinese Lunar New Year on Monday. It has six sites that can test for PM2.5 and 27 that can test for the larger, coarser PM10 particles that are considered less hazardous. The center is expected to buy equipment and build more monitoring sites to enable PM2.5 testing.

Beijing wasn't expected to include PM2.5 in its daily roundups of the air quality anytime soon. Those disclosures, for example "light" or "serious," are based on the amount of PM10, sulfur dioxide and nitrogen dioxide in the air.

Beijing interprets air quality using less stringent standards than the U.S. Embassy, so often when the government says pollution is "light," the embassy terms it "hazardous."

"There has been tremendous amounts of attention in the Chinese media - whichever newspaper you pick up, whichever radio station you listen to, channel you watch - they are all talking about PM2.5 and how levels are so high," said Andrews.

"What has been so powerful is that people are skeptical, and I think rightly skeptical," about the government's descriptions of data, he said.